Chemical mechanical polishing apparatuses (CMP apparatuses) are commonly used for the planarization of silicon wafers. In one type of CMP apparatus, a rotating pad is placed in contact with a rotating wafer and the pad is moved back and forth laterally relative to the rotating wafer. Additionally, a polishing slurry is forced into a gap between the wafer and the pad. The slurry is typically an aqueous solution that carries a high concentration of nanoscale abrasive particles. The slurry can play a number of critical roles in the polishing of the wafer. For example, the chemical composition of the slurry can after the surface properties of the wafer, soften the wafer surface and make it amenable to material removal. Further, the abrasive particles in the slurry remove material from the wafer surface by cutting nanoscale grooves in the wafer surface.
Some in the industry believe that most of the material removal occurs when pad asperities on the pad are in contact with the wafer, trapping slurry particles between them. The asperities push the particles into the wafer surface and drag them along so the abrasive particles act as nanoscale cutting tools.
Designers are constantly trying to improve the accuracy and efficiency of CMP apparatuses. For example, if the force applied by the pad against the wafer is not uniform, the material removal rate will not be uniform. Additionally, if the force applied by the pad against the wafer is not precisely controlled, the planarity of the wafer and accuracy of the CMP apparatus will be diminished.